Format & Content

Daily Product (08_D3) File Format Basics

MODIS Atmosphere Daily (08_D3) product files are stored in Hierarchical Data Format (HDF). HDF is a multi-object file format for sharing scientific data in multi-platform distributed environments. HDF files should only be accessed through HDF library subroutine and function calls, which can be downloaded from the HDF web site.

Each of the global gridded statistical fields within the HDF file is stored as a Scientific Data Set (SDS). L3 Atmosphere products contain hundreds of 1°×1° global gridded Scientific Data Sets (SDSs) or statistics derived from the L2 products of Aerosol (04_L2), Water Vapor (05_L2), Cloud (06_L2), and Atmospheric Profile (07_L2). L3 SDSs derived from Cloud Mask (35_L2) related data are actually read from (duplicated in) the Cloud (06_L2) product, therefore the L2 Cloud Mask (35_L2) product is never read (directly) into L3. The grid domain or size is 360x180 pixels, which corresponds ot 360 degrees of longitude and 180 degrees of latitude.

The SDS Inventory Table linked below shows the full set of statistics computed for each parameters -- noted by a colored circle (dot) in the table.

Derived from the 04_L2 SDS "Optical_Depth_Land_And_Ocean". Note that the land part of this SDS is taken from the 04_L2 SDS Corrected_Optical_Depth_Land at 0.55 microns and the ocean part of this SDS is taken from the 04_L2 SDS Effective_Optical_Depth_Best_Ocean (a parameter that is not propagated into L3 by itself).

Derived from the 05_L2 SDS "Water_Vapor_Near_Infrared" and aggregated into a clear or cloud category from the 05_L2 SDS "Quality_Assurance_Near_Infrared".

Parameters derived from the L2 Cloud (06_L2) Product

Cirrus Detection

Cirrus_Reflectance
Contrail_Reflectance

Derived from the 06_L2 SDS "Cirrus_Reflectance" and aggregated into a cirrus or contrail category from the 06_L2 SDS "Cirrus_Reflectance_Flag".

Cirrus_Fraction_SWIR
Contrail_Fraction_SWIR

Derived from the 06_L2 SDS "Cirrus_Reflectance_Flag" and aggregated into a cirrus or contrail category from the 06_L2 SDS "Cirrus_Reflectance_Flag". This flag differentiates between BadData, Non-Cirrus, Cirrus, or Contrail at 1km resolution.

Derived from the 06_L2 SDS "Cloud_Phase_Infrared" and aggregated into Day or Night categories from the 06_L2 SDS "Cloud_Mask_5km". The bins of the Histogram Counts statistic corresponds to clear (1st bin), liquid water clouds (2nd bin), ice clouds (3rd bin), undetermined phase clouds (4th bin). (Note there are no mixed phase clouds in C6 and later).

Cloud_Fraction
Cloud_Fraction_Day
Cloud_Fraction_Night

This is the primary L3 Cloud Fraction parameter from Cloud Mask. This is derived from the 06_L2 SDS "Cloud_Fraction" and aggregated into Day or Night categories using the day/night flag from the 06_L2 SDS "Cloud_Mask_5km".

Cirrus_Fraction_Infrared
High_Cloud_Fraction_Infrared

Derived from the Cirrus and High Cloud Flags in the 06_L2 SDS "Quality_Assurance_5km". The numbers computed are actually ratios of cirrus clouds (or high clouds) to all clouds. They are not true cloud fractions, where clear sky pixels are accounted for in the denominator. (Note that in the upcoming Collection 006, these numbers will change to true cloud fractions where clear sky pixels will be accounted for in the denominator.)

Derived from the 07_L2 SDS's "Water_Vapor, "Water_Vapor_Low", and "Water_Vapor_High", respectively. The keywords "Low" and "High" refer to the lower and higher portions of the atmospheric column. Note that the parameter "Water Vapor" is also refered to as "Precipitable Water".

Daily Product (08_D3) Statistic List

A number of statistical summaries are computed, depending on the science parameter being considered. Statistics for a given Level-2 measurement might include:

Simple Statistics

Mean

Minimum

Maximum

Standard_Deviation

QA Weighted Statistics

QA_Mean

QA_Standard_Deviation

Histograms of the quantity within each grid box

Histogram_Counts

Histograms of the confidence placed in each measurement

Confidence_Histograms

Fraction of pixels that satisfy some condition (e.g. cloudy, clear)

Fraction

Total number of observations (pixels) within each grid box

Pixel_Counts

Parameters of Log-Normal Distributions

Log_Mean

Log_Standard_Deviation

Parameters of QA Weighted Log-Normal Distributions

QA_Log_Mean

QA_Log_Standard_Deviation

Joint Histograms derived from comparing one science parameter to another, statistics may be computed for a subset that satisfies some condition

Joint_Histogram_vs_Effect_Radius

Joint_Histogram_vs_Temperature

Joint_Histogram_vs_Emissivity

Daily Product (08_D3) Statistic Computation Details

Below are more details on the computation of each statistic:

For the daily product, assume that x1, x2, ..., xn represent the retrieved pixel values of a Level-2 parameter over a 1 x 1 degree grid box, Qi is the quality flag for each retrieved pixel value, and wi is the weighting factor (1 for the daily case), then the simple statistics are defined as:

In these equations, (1) will be referred to as the 'regular' mean, (2) as the QA weighted mean, (3) as the regular standard deviation, (4) as the QA weighted standard deviation, (5) as the minimum, and (6) as the maximum.

The histograms and joint histograms report the counts of the pixels falling into predetermined numerical intervals.

The pixel counts are used to represent the number of pixels for the parameters which do not have QA flags, while the confidence-histograms-counts are used to represent the number of counts for each parameter that falls within each QA bin (e.g., questionable, good, very good, and total).